Ferroelectric properties in thin film barium titanate grown using pulsed laser depositionDaniel J. R. Appleby, Nikhil K. Ponon, Kelvin S. K. Kwa, Srinivas Ganti, Ullrich Hannemann, Peter K. Petrov,Neil M. Alford, and Anthony O'Neill Citation: Journal of Applied Physics 116, 124105 (2014); doi: 10.1063/1.4895050 View online: http://dx.doi.org/10.1063/1.4895050 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The effect of stress on the dielectric and tunable properties of barium stannate titanate thin films Appl. Phys. Lett. 94, 052902 (2009); 10.1063/1.3073743 Orientation dependent ferroelectric properties in samarium doped bismuth titanate thin films grown by the pulsed-laser-ablation method Appl. Phys. Lett. 89, 032901 (2006); 10.1063/1.2221918 Effect of Li substitution on dielectric and ferroelectric properties of ZnO thin films grown by pulsed-laser ablation J. Appl. Phys. 99, 034105 (2006); 10.1063/1.2169508 Effects of poling, and implications for metastable phase behavior in barium strontium titanate thin film capacitors Appl. Phys. Lett. 85, 5010 (2004); 10.1063/1.1827934 Dielectric properties of pulsed laser deposited films of PbMg 1/3 Nb 2/3 –PbTiO 3 and PbSc 1/2 Nb 1/2 O 3–PbTiO 3 relaxor ferroelectrics J. Appl. Phys. 86, 5179 (1999); 10.1063/1.371497

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Ferroelectric properties in thin film barium titanate grown using pulsed laserdeposition

Daniel J. R. Appleby,1 Nikhil K. Ponon,1 Kelvin S. K. Kwa,1 Srinivas Ganti,1

Ullrich Hannemann,2 Peter K. Petrov,2 Neil M. Alford,2 and Anthony O’Neill11School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne NE1 7RU,United Kingdom2Department of Materials, Imperial College London, Royal School of Mines, Exhibition Road, London SW72AZ, United Kingdom

(Received 1 May 2014; accepted 8 August 2014; published online 24 September 2014)

The characteristics of polycrystalline BaTiO3 metal-insulator-metal capacitors, fabricated using pulsed

laser deposition, are investigated from room temperature to 420 K. The capacitance–voltage

characteristics show ferroelectric behaviour at room temperature, with a phase transition to

paraelectric at higher temperature. However, the permittivity response shows paraelectric behaviour

across all measured temperatures. So BaTiO3 exists here in a mixture of cubic and tetragonal phases.

The BaTiO3 films have a columnar structure, with grain size increasing with film thickness due to

their increasing height but not diameter. This correlates with an increase in remnant polarization. The

results support a size driven phase transition in thin films of polycrystalline BaTiO3. VC 2014AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4895050]

I. INTRODUCTION

Perovskite insulators have attracted particular attention

for applications such as ferroelectric random access memo-

ries (FeRAM),1 dynamic RAM,2 and alternative gate materi-

als for metal oxide semiconductor field effect transistors

(MOSFETs).3 A recent review details their short-term and

future prospects for the nanoelectronics industry, further

proof of the need to integrate this promising material in next

generation devices.4 A particular region of interest in these

materials is the ferroelectric phase, which allows polarization

to be retained in the absence of an applied electric field.

Ferroelectricity has been the subject of numerous past stud-

ies,5–8 and is of most relevance in non-volatile memory

applications due to its ability to retain its charge state when

power is removed. However, the ferroelectric phase is sub-

ject to varying degrees of intrinsic and/or extrinsic influen-

ces, ranging from elemental composition,9 annealing, growth

technique,10 and straining,11 among others.12–16 Therefore, it

is important to understand the quality of perovskites in terms

of their demonstrated ferroelectric properties.

The phase of a perovskite is primarily dictated by tem-

perature and described by the Curie-Weiss law, given by

v ¼ C

T � Tc; (1)

where v is the dielectric susceptibility, T is the operating

temperature, Tc is the Curie temperature, and C the Curie

constant. The phase of a perovskite will transition at a tem-

perature T¼Tc; however, in certain materials such as barium

titanate (BaTiO3, BTO), the phase transition temperature can

be approximately 10 K above Tc.17

From Eq. (1), it follows that by increasing the operating

temperature beyond Tc, the material will show decreasing sus-

ceptibility, and in turn permittivity (v¼ er � 1, where er is the

relative permittivity of the material). In contrast, operating

below Tc should give negative v, which leads to instability,

and hysteretic characteristics ensue. The aforementioned

properties are descriptive of the paraelectric phase when

T>Tc, and the ferroelectric phase in cases when T<Tc. A

key property of a perovskite material is its phase transition

temperature, Tc. For the case of BTO in bulk or powder form,

this temperature is approximately 393 K and where the per-

mittivity is a maximum (Eq. (1)).18 However, experimentally

this property is known to change, particularly when dealing

with thin film perovskites, and the permittivity peak tends to

broaden or shift altogether.19 In the high temperature phase

(T>Tc), the perovskite crystal is cubic due to thermal agita-

tion. Decreasing the operating temperature past the phase

transition point (T<Tc) the perovskite crystal is tetragonal;

long-range coulombic ordering results due to the balance of

the thermal and electrostatic forces, allowing an elongation of

the crystal c-axis and the creation of a built-in permanent

dipole. The tetragonal deformation of a ferroelectric crystal

allows alignment and subsequent reorientation of these per-

manent dipoles through the application of an external electric

field.

A so-called "size driven phase transition" has been stud-

ied previously, specifically on particles20 or freestanding

films22 of perovskite material. It places a critical limit on

crystal grain size, such that a smaller grain structure shifts

the phase transition temperature to lower values. Therefore,

the size driven phase transition leads to materials that are

paraelectric in phase at operating temperatures previously

expected to be in the ferroelectric phase. Numerous studies

on BTO nanoparticles have estimated a critical grain size,

below which no ferroelectric properties are seen, to lie

between 17 and 190 nm [Ref. 20 and references therein]. The

grain size dependence has also been incorporated into

Landau-Ginsburg-Devonshire theory in which grain size was

varied from 50 to 1200 nm in dense BTO ceramics.21 The

results showed progressively lesser tetragonal phase with

0021-8979/2014/116(12)/124105/6/$30.00 VC 2014 AIP Publishing LLC116, 124105-1

JOURNAL OF APPLIED PHYSICS 116, 124105 (2014)

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decreasing grain size, in concurrence with a shift in the phase

transition temperature to lower values as the grain size

reduces. Conversely, a grain size dependence on thin film

PbTiO3 was studied, which showed a transition from multi-

domain to single domain structures at 150 nm grain size or

less.22 The retained tetragonal phase in the single domain

structure exhibited a lack of polarization switching under the

application of an electric field and low permittivity. The

results are attributed to single domain structures in the

smaller grains due to the inherent lack of domain walls and

their subsequent movement. However, this result was meas-

ured on a free-standing film, and hence suffered no influence

from the substrate.

In this study, thin films of BTO deposited by pulsed

laser deposition (PLD) are investigated. The films are grown

on Pt/Ti/SiO2/Si substrates, and investigated in terms of fer-

roelectricity for Si integration applications. We show that

thin films of BTO are in a mixed phase relationship, consist-

ing of the cubic paraelectric phase, in addition to the tetrago-

nal ferroelectric phase. Furthermore, it is shown that the

cubic phase has a tendency to dominate at room temperature.

The dominant cubic phase leads to a measured reduction in

the overall remnant polarization, a common occurrence fre-

quently reported in the literature23,24 in terms of thin film

and grain size dependence. The results confirm a size driven

phase transition in thin films clamped to a substrate with co-

lumnar structures. The experiments indicate that increasing

the heights of the columns by growing thicker films increases

the remnant polarization in the thin films. The remnant polar-

ization increase is due to an increase in grain size as the col-

umns grow in height. With an increase in the film thickness,

the column widths remain approximately 30 nm in size.

II. EXPERIMENT

A BTO thin film of 160 nm thickness was deposited on a

Pt/Ti/SiO2/Si stack using PLD at a growth temperature of

740 �C. Further BTO films of 280 nm and 380 nm were also

deposited for thickness dependence studies. The PLD system

was equipped with a KrF excimer laser from Lambda

Physik. Before deposition, the substrate was heated up to a

deposition temperature of 740 �C. The deposition was per-

formed at a repetition rate of 5 Hz in an oxygen pressure of

100 mTorr. The energy density of the focussed laser beam

on the target was 4 J/cm2. After deposition, the vacuum

chamber was flooded with oxygen up to 500 Torr and the

sample was cooled down to room temperature at a cooling

rate of 10 �C/min. The Pt substrate layer (100 nm) and the Ti

adhesion layer (10 nm) were deposited using electron beam

evaporation. Pt top electrodes with diameters ranging from

50 to 300 lm were then fabricated to create metal-insulator-

metal (MIM) structures, which were used for electrical

measurements.

Small-signal capacitance-voltage (CV) electrical meas-

urements were taken at temperatures ranging from room

temperature (300 K) to 420 K. CV measurements were taken

at 1 MHz and consisted of both forward (negative to positive

sweep) and reverse (positive to negative sweep) biasing to

analyse the BTO film. All remaining measurements were

taken at room temperature. Large-signal hysteresis measure-

ments were taken on a Radiant Precision Premier II. X-ray

diffraction (XRD) spectra were acquired using Panalytical

x’pert and the Raman spectra were acquired by a HORIBA

scientific LabRam using a 514 nm laser. Electrostatic force

microscopy (EFM) images were taken using an NSC 14

Au=Cr cantilever. Atomic force microscopy (AFM) in non-

contact mode was also conducted to verify grain size using a

non-contact high resonance frequency–reflex coating tip.

III. RESULTS

A. Electrical

Figure 1 shows the CV characteristics for a MIM capaci-

tor with a BTO insulating layer of 160 nm thick performed at

two different temperatures, 300 K and 420 K. Measurements

performed at both temperatures should exhibit ferroelectric

and paraelectric characteristics, respectively, since the phase

transition temperature Tc¼ 393 K. At 300 K, measurements

show two distinct curves, with the applied forward and

reverse bias direction shown by the arrows. The curves indi-

cate a two peak (“butterfly”) trend in the measurement, and

this is attributed to the polarization switching in a ferroelec-

tric, i.e., the material displays directionally dependent char-

acteristics with applied bias through its hysteretic

polarization (P 6¼ 0 at V¼ 0). These results are indicative of

ferroelectricity in BTO measured at 300 K.

The 160 nm thick BTO capacitor was measured at 420 K

in order to induce a phase transition. The CV characteristics

shift as the measurement temperature increases past the bulk

value of Tc (Fig. 1). The intermediate measurements (not

shown) between temperatures 300 K and 420 K evolve gradu-

ally with increasing temperature towards the characteristics

seen at 420 K. The double capacitance peaks observed at

300 K converge to a single peak and the butterfly trend is lost.

The high temperature measurement is indicative that a phase

transition has taken place in the BTO film such that each unit

cell is now cubic and no longer retains any polarization.

Effective permittivity is determined for the 160 nm thick

BTO capacitor using the parallel-plate capacitor relationship

of the MIM structure. The permittivity response in a

FIG. 1. Capacitance-voltage measured on the Pt/BaTiO3 (160 nm)/Pt capaci-

tor at 300 K and 420 K. Electric field is taken as V/tox, where tox is the thick-

ness of the BaTiO3 film.

124105-2 Appleby et al. J. Appl. Phys. 116, 124105 (2014)

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perovskite may show evidence of a phase transition in ac-

cordance with Eq. (1). When the ferroelectric transitions into

the paraelectric phase, the permittivity will increase, reach-

ing a maximum at the phase transition temperature, 393 K.18

From the capacitance measurements shown in Fig. 1, it is

expected that a peak permittivity will be seen at approxi-

mately 400 K. This would suggest a phase transition in the

160 nm thick BTO film.

Figure 2 shows the extracted effective permittivity vari-

ation with temperature from capacitance measurements

using the parallel-plate relationship. Increasing the tempera-

ture towards 400 K, there is an absence of any peak in the

permittivity. In contrast, measurements on freestanding BTO

showed permittivity increasing sharply from approximately

380 K, and reaching a peak Curie anomaly at 400 K, demon-

strating its ferroelectric nature.19 Fig. 2 also shows that per-

mittivity increases when the film thickness was increased

from 160 nm to 280 nm. The permittivity further increases

for the 380 nm thick BTO film. The trend shown here is indi-

cating a paraelectric characteristic for all the three thick-

nesses where the permittivity is decreasing with increasing

temperature, as described by Eq. (1). The result suggests that

the phase transition has shifted to a lower temperature,

whereby the paraelectric phase is induced at T<Tc¼ 393 K.

However, the permittivity analysis contradicts the CV meas-

urements in Fig. 1, which shows a convergence of the peak

capacitance points when the temperature is increased to

420 K. Moreover, an increase in permittivity with increase in

thickness points toward a change in the crystal structure of

the BTO film with thickness.

The electrical results for the BTO capacitor disagree in

terms of the properties displayed: the butterfly CV trend in

Fig. 1 shows evidence for the tetragonal phase, while the per-

mittivity response with temperature in Fig. 2 suggests a para-

electric phase based on Curie-Weiss law. The results lead to a

conclusion that the film is in a mixed phase condition. The

tetragonal part is responsible for the butterfly CV measure-

ments, while the cubic phase dictates the permittivity

response with temperature. The cubic phase is thought to be

the dominating contribution due to the overriding paraelectric

response to permittivity when the temperature increases.

High concentrations of the cubic phase will manifest as mini-

mal remnant polarization, which is apparent in a number of

studies.23 The measured permittivity is an effective permittiv-

ity, which has contributions from both cubic and tetragonal

phase. An increase in effective permittivity with thickness

can then be manifested as an increase in percentage of tetrag-

onal phase with increase in thickness of the film. Frequency

dependence on the capacitance was also measured for values

ranging from 1 kHz to 1 MHz. The reduction in capacitance

with increase in frequency was found to be negligible. A

nearly constant capacitance for the measured frequency range

points to a defect free film structure. Minimal remnant polar-

ization in the BTO capacitors is investigated with large-

signal hysteresis measurements in Sec. IV.

B. Material

In order to investigate the material characteristics and

mixed phase properties of the BTO film, EFM, XRD, and

Raman spectra were taken at room temperature. Before the

EFM images were taken, in order to polarize the BTO, �5 V

was applied to the bottom Pt electrode and a 4 lm � 4 lm

area was scanned in contact mode. The EFM images were

taken across a 6 lm � 6 lm area which covers the previously

polarized area. The alternating modulation applied to the tip

had an amplitude of 0.8 V, and a frequency of 20 kHz.

XRD spectra (Fig. 3) of the 160 nm thick film is per-

formed to investigate the crystallinity and to determine

growth orientation. Results from the 280 and 380 nm thick

BTO films are also shown and discussed in Sec. IV. The

XRD peak intensity increased with increase in film thickness

since the intensity is proportional to the volume of diffract-

ing planes. From the results, it can be concluded that the

BTO film is polycrystalline with random orientation, as evi-

dent from the numerous diffraction peaks in the spectrum.

Therefore, the BTO film consists of randomly oriented grains

with intersecting grain boundaries. Peak splitting in XRD

spectra may show evidence for the tetragonal phase in a

FIG. 2. Effective permittivity measured on the Pt/BaTiO3/Pt capacitor as a

function of increasing temperature. Permittivity is extracted from the

parallel-plate capacitor relationship.

FIG. 3. XRD spectra of the BTO films with varying film thickness.

Orientations of BTO are labelled with remaining contributions originating

from Si, Pt, and Ti.

124105-3 Appleby et al. J. Appl. Phys. 116, 124105 (2014)

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perovskite crystal. The data presented in Fig. 3 does not sig-

nify any peak splitting; however, this is likely to be due to

the resolution of the XRD measurement and as such the

results do not factor into the conclusion about the phase of

the BTO.

Further understanding of the structure of the BTO film

can be gained through the TEM imaging in Fig. 4 for the

160 nm thick sample. The image in Fig. 4(a) shows the full

BTO film thickness situated between the dark contrasting Pt

electrodes. It is apparent that the film has grown in a colum-

nar structure, originating from the initial bottom Pt electrode.

Each column spans the full film thickness and they coalesce

towards the top Pt electrode; therefore, each column height is

the full thickness of the given film. From Fig. 4(a), each col-

umn width is estimated at 30 nm. Also shown in Fig. 4(b) are

high resolution TEM images of the individual BTO columns.

The results show that, taking into account the influence of

Moir�e fringes, each column is a uniform grain of single orien-

tation, leading to grain boundaries at the column edges.

The EFM technique is applied to study any remnant

polarization and domain structure in the BTO film. Fig. 5

shows the EFM scan of 6 lm � 6 lm area on the surface of

the 160 nm thick BTO film. The polarized 4 lm � 4 lm

region appears as a bright square, in sharp contrast with the

outer un-polarized area. Colour variation in the polarized

region as shown in Fig. 5 also indicates that the film is non-

uniformly polarized. The result is further highlighted by a

magnified scan of 1 lm � 1 lm on the polarised area, clearly

showing highly contrasted regions. The dark regions indicate

non-polarizable domains, whereas white regions show the

polarized domains. The cubic phase in BTO does not retain

the spontaneous polarization upon the removal of an external

field, whereas in the ferroelectric tetragonal phase it does.

The co-existence of polarizable and non-polarizable domains

confirms the presence of both cubic phase and tetragonal

phase in polycrystalline BTO, in adherence with the previous

results.

Raman spectra of thin film and bulk ceramic BTO from

100 cm�1 to 800 cm�1 at room temperature are given in Fig.

6. BTO reveals Raman active bands around 250, 306, 520,

and 720 cm�1. A sharp peak at approximately 300 cm�1 and

720 cm�1 is a characteristic of tetragonal BTO.25 The

FIG. 4. TEM image of the 160 nm thick BTO sample. (a) Image shows the

cross section of the full film thickness situated between the dark contrasting

Pt electrodes. (b) Images show high resolution TEM highlighting individual

columns.

FIG. 5. EFM image of polycrystalline BTO at 160 nm thick (a). BTO is non-

uniformly polarized. High resolution scan shows that it is a mixture of polar-

izable and non-polarizable domains (b).

FIG. 6. Raman spectra of polycrystalline BTO as a function of increasing

thickness. Dashed region shows BTO has a reduced peak intensity around

300 cm�1 and 720 cm�1 indicating presence of both cubic and tetragonal

phases. Inset highlights that the peak intensity at 300 cm�1 increases with

increasing film thickness.

124105-4 Appleby et al. J. Appl. Phys. 116, 124105 (2014)

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300 cm�1 and 720 cm�1 peak disappears as the phase changes

from tetragonal to cubic, whereas the 250 and 520 cm�1 peaks

remain the same. Data for the 160 nm thick BTO film pre-

sented here show a reduced intensity peak around 300 cm�1

and 720 cm�1 compared to a bulk ceramic. The intensity of

the Raman peaks is not affected by crystallographic orienta-

tion. Therefore, the reduction in the intensity of the Raman

peak corresponding to the tetragonal phase (300 cm�1) indi-

cates the presence of both cubic and tetragonal phases, further

supporting the results shown previously. Analysis of the

Raman spectra of all BTO thicknesses studied here is given in

Sec. IV.

IV. DISCUSSION

Based on the previous results, thin film BTO is observed

to be in a mixed phase condition comprising cubic and tet-

ragonal parts. The butterfly CV trend and bright contrasting

regions in EFM confirm the existence of the tetragonal

phase. The permittivity response with temperature, dark con-

trasts in EFM, and lack of any Raman peak at 300 cm�1

show evidence for the cubic phase. The studies discussed

were for the 160 nm film thickness, which showed polycrys-

talline, randomly oriented grains in a columnar structure.

The discussion will now turn to the thickness dependence of

the films and 280 nm and 380 nm thick films will be com-

pared with the results of the 160 nm thick film.

The remaining XRD spectra for the 280 nm and 380 nm

thick films are presented in Fig. 3. The results are unchanged

to those described previously for the 160 nm thick film. Each

film thickness shows a polycrystalline and randomly oriented

grain structure. Therefore, an increasing film thickness does

not impact the texture or crystallinity of the BTO films.

The Raman spectra for the 280 nm and 380 nm thick

films are studied, particularly the peak at 300 cm�1 and

720 cm�1 indicative of the tetragonal phase. Fig. 6 shows the

full range of each spectrum, and the inset reflects the magni-

fied region of the peak of interest. As the film thickness is

increased, both the 300 cm�1 and 720 cm�1 peaks are seen to

increase in intensity. These results mean that the contribution

from the tetragonal phase increases as the thickness of the

films increase. Consequently, an increasing film thickness

should lead to an increase in remnant polarization.

Figure 7 shows a large-signal hysteresis measurement

on the 380 nm thick sample. The inset in Fig. 7 also shows

the remnant polarization measured in each sample to corrob-

orate the thickness dependent Raman results discussed

above. From the results, a clear thickness dependence is evi-

dent on the remnant polarization, which increases with BTO

film thickness, leading to a confirmation of the Raman

results. Therefore, it is initially inferred that an increase in

the film thickness has increased the grain size of the film. In

accordance with the size driven phase transition, the larger

grains in the thicker films will be tetragonal and contribute

to the increased remnant polarization. Thus, the concentra-

tion of the cubic phase in these mixed phase films will be

reduced in thicker films.

In order to verify any changes in the grain structure of

the films, AFM topography scans are conducted and shown in

Fig. 8. Each image shows an unchanged topography, which

demonstrates an unchanging grain diameter as the film thick-

ness is increased. However, from the TEM images of the

160 nm thick film in Fig. 4, BTO is known to grow in a co-

lumnar structure. Therefore, as the film thickness is increased

the height of each individual column increases, while the di-

ameter of each column stays constant as measured in the

AFM scans. The increasing remnant polarization with film

thickness is therefore due to an increase in the volume of each

individual column as they grow in height. The increase in vol-

ume will favour the tetragonal phase whereby a larger number

of dipoles are allowed to align along the polar axis, leading to

the increase in remnant polarization. Ferroelectricity as a co-

operative phenomenon favours an increase in the number of

dipoles available for interaction, leading to an energetically

stable ferroelectric phase. Previous studies discuss the number

of required dipoles in terms of a correlation volume.26 In the

polycrystalline films with no preferred orientation, a similar

effect should be seen if the diameter of the columns increases,

which would be reflected in a changing topography in AFM

scans. An increasing contribution from the ferroelectric phase

(tetragonal phase) with thickness is also evident from the tem-

perature dependent permittivity measurements. A large

increase in permittivity with thickness is due to the percentage

increase in tetragonal phase in the total volume.

V. SUMMARY

Analysis of thin film BTO integrated with Si on Pt/Ti/

SiO2/Si substrates showed evidence of a mixed phase prop-

erty in the films. The mixed phase film explains the low rem-

nant polarization in thin film BTO as reported in the

literature.23 Large-signal measurements showed that

FIG. 7. Large signal hysteresis measurements of the 380 nm thick BTO film.

Inset shows the measured remnant polarization as a function of film

thickness.

124105-5 Appleby et al. J. Appl. Phys. 116, 124105 (2014)

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increasing the film thickness led to an increase in the rem-

nant polarization. The Raman spectra also confirmed that an

increase in the film thickness correlated with increasing peak

intensities at 300 cm�1, which is indicative of the tetragonal

phase. XRD results showed that the crystallinity and texture

stay constant with increasing film thickness. Furthermore,

AFM and TEM imaging showed that the films grow in a co-

lumnar structure in which an individual column stays at a

constant diameter as the film thickness increases. The results

are such that as the films are grown at increasing thicknesses,

the variable in terms of film structure is the increase in the

height of each individual column.

The size driven phase transition sets a critical grain size

in order for a material to display ferroelectric properties. In

dense BTO ceramics, the tetragonal phase was shown to be

progressively less prominent.21 Here, we show that for a thin

film BTO grown in a columnar structure, the height of the

individual column acts to increase the remnant polarization

due to the increasing contribution from the tetragonal phase.

In each film, the diameter of the columns stays at an approxi-

mate 30 nm in size.

ACKNOWLEDGMENTS

This work was supported by EPSRC and Intel Ireland.

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